[Media] Allow a11y interactions to change seekbar position.

Note that the seekbar UI can now get a bit janky if
you have TalkBack on and you very quickly swipe up or down. We can file
a new bug for that if we decide it needs fixing.

Fixes: 216254595
Fixes: 216254099
Test: SeekBarViewModelTest
Test: manual: Turn TalkBack on, select the seekbar, and verify you can
swipe up/down to change the track position.
Test: manual: Verify seeking without TalkBack on still works.

Change-Id: I890e4f396fa704047c5593fff20c8924b890fc75